A NASA team at GSFC (Goddard Space Flight Center) in Greenbelt, Maryland, is collaborating with OSU (Ohio State University) and NASA/JPL (Jet Propulsion Laboratory) in Pasadena, California, to build and launch a new CubeSat mission that will test next-generation techniques for detecting and discarding RFI (Radio Frequency Interference). Funded by NASA's InVEST (In-Space Validation of Earth Science Technologies) program, the CubeRRT project specifically will evaluate a specialized digital-based spectrometer equipped with sophisticated algorithms that can detect and mitigate the radio interference that spills over and ends up as noise in scientific data. 1)

Goddard is charged with developing the instrument's front-end microwave electronics and overseeing the instrument's integration onto the spacecraft. JPL, meanwhile, is building the instrument's backend digital electronics. The Wallops Flight Facility on Virginia's Eastern Shore is handling ground-system design and operations, while Ohio State's Joel Johnson is leading this effort. In addition, Ohio State is implementing the dual-helical antenna and procuring the spacecraft bus from the Boulder, Colorado-based BCT (Blue Canyon Technologies). 2)3)

RFI is a real problem for Earth science observations. To that end Blue Canyon Technologies will build the CubeRRT for Ohio State University to provide more accurate measurements and radiometers. The goal is to identify and mitigate RFI for spaceborne microwave radiometers. As spectrum usage has increased by the communications industry, RFI has become a problem for science missions. CubeRRT will build on the success of RFI mitigation techniques developed for NASA's SMAP (Soil Moisture Active/Passive) mission, but at higher frequencies, 6 - 40 GHz, instead of SMAP's 1.4 GHz. Mitigation at these higher frequencies enables scientists to continue using radiometry for high quality Earth science measurements. 4)5)

Background: The cacophony of signals constantly at play on Earth - from radio traffic to cell phones and other communications systems – ultimately creates the white noise of human technology in action. In space, however, this cacophony is getting in the way of some important Earth science research - and it's only getting worse as human technology advances. Joel Johnson, chair and professor of the Department of Electrical and Computer Engineering at OSU, is now leading a NASA program to help navigate the noise. Since 2001, his team at OSU has focused on detecting and discarding man-made RFI from the Earth's naturally fluctuating microwave signals. The technology is imperative for future satellite missions using microwave radiometry to observe Earth's properties. 6)

Johnson's team contributed similar technologies for NASA's SMAP (Soil Moisture Active Passive) satellite, which launched in January 2015. SMAP is now helping scientists create the most detailed global maps of soil moisture to date, improving understanding of Earth's water and carbon cycles, as well as the ability to manage water resources.

RFI subsystems for higher frequency microwave radiometry over the range 6-40 GHz, however, require a larger bandwidth, so that the capabilities of RFI mitigation backends in terms of bandwidth and processing power must also increase. To date, no such wideband subsystem has been demonstrated in space for radiometers operating above 1413 MHz.

The enabling technology is a digital FPGA (Field Programmable Gate Array)-based spectrometer with a bandwidth of 1 GHz or more and capable of implementing advanced RFI mitigation algorithms such as the Kurtosis and cross-frequency methods. This technology has a strong ESTO heritage, with the algorithms developed and demonstrated via the IIP (Instrument Incubator Program) and wideband backends developed under other ESTO support. The digital backend is currently at TRL 5, having been successfully tested in an RFI environment, and can be ported easily to a flight-ready firmware. Though the technology can be demonstrated for any frequency band from 1 to 40GHz, the CubeRRT project will integrate the backend with a wideband radiometer operating over a 1 GHz bandwidth tunable from 6-40 GHz to demonstrate RFI detection and mitigation in important microwave radiometry bands. Along with a wideband dual-helical antenna, the payload will be integrated with a 6U CubeSat to demonstrate operation of the backend at TRL 7. The payload is expected to operate at a minimum duty-cycle of 25% to be compatible with spacecraft power capacity. Although the spatial resolution to be achieved will be coarse (due to the limited antenna size possible), the goal of demonstrating observation, detection, and mitigation of RFI is achievable in this configuration.

Figure 1 illustrates the 6-40 GHz portion of the spectrum, along with the frequency ranges used in several past radiometer missions; the sensitivity to environmental effects in each of these frequencies is also shown in the lower curves. Passive microwave observations are allocated primary-use only in a small number of bands (those shown as green vertical bars), with shared-use in those marked in yellow. Due to the high sensitivity of radiometer measurements, shared allocations. offer little protection from RFI corruption, as amply demonstrated in numerous past missions. As active sources expand over larger areas and occupy additional spectrum, it will be increasingly difficult to perform radiometry without an RFI mitigation capability. Co-existence in some cases may be possible provided that a subsystem for mitigating RFI is included in future systems (Ref. 5).

Figure 1: Frequency ranges allocated for microwave radiometer observations (green=primary, yellow=shared) in the 6-40 GHz range. Spectral ranges used by several past missions also indicated, as well as curves of sensitivity to environmental parameters vs. frequency (CubeRRT collaboration)

• Operate for one year at 25% duty cycle to acquire adequate RFI data.

Parameter

SMAP

Future

Bandwidth

20 MHz

100's of MHz in each channel

RFI processing on ground ?

Yes (limited downlink volume)

Not possible (downlink volume too high)

RFI processing on-board spacecraft ?

No: not necessary

Yes: only way to address RFI challenge for future systems

Table 1: Challenges in current and future missions

Spacecraft:

CubeRRT is a 6U CubeSat of size 20 cm x 30 cm x 10 cm to be built by BCT (Blue Canyon Technologies) of Boulder, CO for OSU. BCT will integrate the CubeRRT payloads with the 6U spacecraft bus and perform environmental testing of the complete spacecraft. The spacecraft will be operated from BCT's Mission Operations Center located in Boulder, Colorado. BCT's 6U spacecraft is a high-performance CubeSat that includes an ultra-precise attitude control system that allows for accurate knowledge and fine-pointing of the satellite payload (Ref. 4).

CubeRRT is expected to operate with a duty-cycle of 30%. CubeRRT will mostly be operational over landmasses where the occurrence of RFI is expected to be significantly higher. CubeRRT is designed for 12 months of commissioning and operations. Once the CubeRRT mission has met all of its validation goals, the proven technology can be included in NASA's future radiometry missions to ensure they can continue to collect high-quality Earth-observing data – data that will help researchers answer scientific questions about our planet and better understand our home.

Figure 4: The CubeRRT satellite and Blue Canyon Technologies team members with Principal Investigator Joel Johnson (far left) of The Ohio State University (image credit: Blue Canyon Technologies) 7)

Launch: The CubeRRT satellite was launched on 21 May 2018 (08:44 UTC) on the Cygnus CRS-9 flight of Orbital ATK (OA-9E), ELaNa-23 flight of NASA to the ISS. The launch vehicle was Antares 230 and the launch site was MARS (Mid-Atlantic Regional Spaceport) LP-0A, Wallops Island, VA, USA. 8)

• AeroCube-12A and -12B, a pair of 3U CubeSats of the Aerospace Corporation, El Segundo , CA, to demonstrate a the technological capability of new star-tracker imaging, a variety of nanotechnology payloads, advanced solar cells, and an electric propulsion system on on one of the two satellites (AC12-B).

• EnduroSat One, a 1U CubeSat of Bulgaria, developed by Space Challenges program and EnduroSat collaborating with the Bulgarian Federation of Radio Amateurs (BFRA) for the first Bulgarian Amateur Radio CubeSat mission.

Figure 5: First light on 5 September 2018. On-board filtering requires 99% less data to be downlinked (image credit: OSU)

• October 2, 2018: The debut data from The Ohio State University's first satellite transmitted back from orbit, and the results look promising for future scientists studying the Earth. 11)

- "The data we received on Sept. 5 confirmed successful real-time on-board removal of RFI (Radio Frequency Interference) from CubeRRT's measurements," Johnson said. "This was the primary goal of the mission, so it was a great feeling to know we had reached this important milestone and that our little satellite was making big accomplishments up in space."

- From July through early September, tests and checkout of the CubeRRT spacecraft communications, power and other subsystems were performed before turning on the RFI processor in September. The first light dataset was acquired over the Pacific Ocean on Sept. 5 during a 10-minute period of measurements.

- In terms of those measurements, the team explained how CubeRRT observes a 1 GHz wide portion of the electromagnetic spectrum, tuned over 10 different bands commonly used for Earth observing microwave radiometers. The total noise power in the 1 GHz bandwidth varied with time as differing portions of the ocean were observed. Because the observations also included a low level of RFI, the original data without RFI correction had a power level higher than the true Earth emitted power. CubeRRT's RFI processor produced the corrected noise temperature on board the spacecraft that eliminated RFI contributions. Validation of CubeRRT's processing was also performed on the ground, and a high level of agreement validated the on board processing.

- Christopher Ball, a research scientist at Ohio State's ElectroScience Lab, said by performing RFI correction onboard the spacecraft, CubeRRT eliminates the requirement to downlink additional data to the ground to perform RFI processing. This was the technological barrier the team hoped the new advancements could solve, and they were correct.

- According to the results, he said, the reduction in data volume is greater than a factor of 100 times, which makes the incorporation of crucial RFI processing feasible for future Earth observing radiometers.

- "We're now continuing to operate CubeRRT's RFI processor over longer time periods to demonstrate long-term operations," Johnson said. "The team is very excited to see what comes next for CubeRRT and to transition this new technology into future Earth observing satellites."

• July 13, 2018: NanoRacks successfully completed the 14th CubeSat Deployment mission from the Company's commercially developed platform on the International Space Station. Having released nine CubeSats into low-Earth orbit, this mission marks NanoRacks' 185th CubeSat released from the Space Station, and 217th small satellite deployed by NanoRacks overall. 12)

- The CubeSats deployed were launched to the Space Station on the ninth contracted resupply mission for Orbital ATK (now Northrop Grumman Innovation Systems) from Wallops Island, Virginia in May 2018.

- NanoRacks offered an affordable launch opportunity, payload manifesting, full safety reviews with NASA, and managed on-orbit operations in order to provide an end-to-end solution that met all customer needs.

- The CubeSats mounted externally to the Cygnus spacecraft from the May 2018 launch are scheduled to be deployed on Sunday, July 15th, pending nominal operations.

Sensor complement:

The CubeRRT payload has three critical pieces of technology, a wideband antenna unit, a radiometer front-end (RFE) unit, and a radiometer digital back-end (RDB) that performs the on-board detection and filtering of RFI. The main objective of the CubeRRT mission is to demonstrate the RFI mitigation technology on a flight-ready hardware in space, increasing the technology readiness level (TRL) from 6 to 7. CubeRRT is designed to make wideband measurements over the whole 6 to 40 GHz range, but the prime mission objective is to demonstrate RFI mitigation over ten "golden" frequency bands that are allocated to Earth observation bands. The Ohio State University leads the CubeRRT mission. The algorithm validating back end technology is built at Jet Propulsion Laboratory, California Institute of Technology and the radiometer front end is built at NASA Goddard Space Flight Center. 13)

The dual-element antenna system is composed of a conical antenna (three circularly polarized tapered helical antennas, 6-40 GHz). The antennas are being designed, developed and tested at The Ohio State University. The series of antenna are necessary to provide sufficient gain over a wide range of frequencies from 6 to 40 GHz. The current design provides a gain of 12 dBi at 6 GHz and 21 dBi at 40 GHz.

The CubeRRT RFE (Radiometer Front-End) is designed to sweep from 6 to 40 GHz with a 1 GHz bandwidth being injected into the RDB. The radiometer is a single tunable superheterodyne receiver. At the front-end of the radiometer the RFE has a four-position switch to choose between the three helical antennas as well as a reference load for calibration. The RFE contains front-end wideband coaxial low noise amplifiers for pre-amplification. The RFE also contains a coupled wideband noise-source to for full internal calibration of the radiometer. The RFE achieves frequency tuning via a phased-locked oscillator (PLO) and sub-harmonic image rejection (IR) mixer. The design allows flexibility between choosing upper and lower side-bands to completely cover the 6 to 40 GHz regime. The architecture sacrificed radiometer performance to meet within the size, mass and power requirements of the 6U system.

The RDB (Radiometer Digital Backend ) of NASA is designed to digitize a 1 GHz bandwidth signal and perform advanced digital signal processing algorithms on an on-board FPGA for RFI mitigation. The RDB ADC is capable of ingesting the IF signal produced by the RFE from 1-2 GHz aliased region. The FPGA proceeds to produce a 128 frequency spectra of the incoming signal using a front-end polyphase filter-bank. The output of the 128 channel spectra then undergoes gain adjustment to account for the non-uniform pass-band shape of the RFE signal. The higher order statistical moments of the data per channel are calculated (2nd and 4th moment) as a pre-cursor to RFI detection and mitigation. The second moment is uncalibrated power of the signal. The first RFI detection algorithm applied is a simple threshold detection algorithm across the spectra to detect frequency outliers. The second RFI detection algorithm is more advanced and uses the fourth and second moments to calculate kurtosis of the signal as a test of normality. Any signal that deviates from normality is flagged as being corrupted by RFI. The flags of the two algorithms are combined and the power in each frequency bin is summed to produce mitigated and unmitigated accumulated power. CubeRRT will downlink all 128 channels to verify performance of mitigated and unmitigated uncalibrated power outputs. Most of the thresholds, coefficients, gain-adjustment values within the RDB are updatable from the user perspective. Figure 6 shows the RFE and RDB subsystems mounted to a baseplate for integrated payload testing.

NanoRacks CubeSats are delivered to the International Space Station (ISS) already integrated within a NRCSD (NanoRacks CubeSat Deployer) or NanoRacks DoubleWide Deployer (NRDD). A crew member transfers each NRCSD/NRDD from the launch vehicle to the Japanese Experiment Module (JEM). Visual inspection for damage to each NRCSD is performed. When CubeSat deployment operations begin, the NRCSD/NRDDs are unpacked, mounted on the JAXA Multi-Purpose Experiment Platform (MPEP) and placed on the JEM airlock slide table for transfer outside the ISS. A crew member operates the JEM Remote Manipulating System (JRMS) – to grapple and position for deployment. CubeSats are deployed when JAXA ground controllers command a specific NRCSD. 14)

CubeRRT operations are commanded based on a scheduler simulation tool developed by OSU (Ohio State University). This tool may be used to develop algorithms for power cycling and frequency tuning, which propagates over the orbital lifetime to predict and optimize CubeRRT measurement results. The scheduler provides information such as the duration until mission-level requirements are fulfilled, radiometry coverage maps, long-term battery depth-of-discharge (DOD), and payload data. In addition, the scheduler may be used to automate the process of generating payload command sequences uplinked regularly to the spacecraft during operations.

CubeRRT's mission goals of evaluating on-board detection and filtering of RFI, coupled with the limited solar cell and battery capacity of a 6U CubeSat, bring about unique challenges for payload operations. These challenges primarily reside with regard to scheduling the power cycling and frequency tuning of the payload. In contrast to other radiometer missions, which typically aim to gather brightness temperature information of the entire Earth's surface, CubeRRT's power budget allows operation only at a duty cycle of approximately 30%. The system prioritizes operation of the payload at locations of known RFI sources and over landmasses where RFI is more likely to occur. 15)

To properly model CubeRRT operations, the scheduler simulates the power system, telemetry buffers, RFI coordinates, and orbital propagation models. The power system state is modelled with knowledge of the available energy from CubeRRT's solar cells, the known power draw from the satellite bus and payload subsystems, and the battery capacity. Telemetry and payload data buffers are monitored and downlinked at a known rate to a ground station at Wallops Flight Facility (WFF) to predict and prevent buffer overflow conditions. The current set of RFI locations is generated from existing radiometry datasets produced by previous nadir-observing missions (Jason and TRMM).

The general algorithm for power cycling divides payload operation into three 10 minute blocks within a ~90 minute orbit, resulting in a 30% duty cycle as illustrated in Figure 7. The payload orients the three blocks by first prioritizing observation at known RFI locations, followed by land observation. If RFI and land are exhausted for a particular orbit, operation during maximum solar availability is then selected. If the chosen sequence exceeds a battery depth of discharge threshold, the power sequence reverts to operating in peak sun. While the payload is operating, the payload sequentially sweeps through the ten radiometer bands in ~15-second increments.

This scheduling algorithm was propagated over the one year expected mission lifetime, with results shown in Figure 8. The payload exhibits an RFI efficiency of approximately 98%, defined as the percentage of time the payload operates when known RFI points are visible within the radiometer's footprint. The payload will spend approximately 65% of its operational time observing landmasses. It will observe approximately 99% of available land (excluding the South Atlantic Anomaly around eastern South America) within the latitude limits of an ISS deployed orbit at least once over the year of operation.

Figure 8: CubeRRT coverage at 6.8 GHz following one year of operations (image credit: CubeRRT Team)

The general workflow for uplinking commands and downlinking telemetry and payload data is shown in Figure 9. OSU will generate daily Planning Action Lists (PALs) based on running the CubeRRT scheduler. The PALs are sent to BCT to develop and validate the spacecraft command list, which is then sent to WFF for uplink to CubeRRT on the next available pass. Downlinked data is sent from WFF to BCT, where it is parsed and archived accordingly. OSU then processes the payload data and telemetry to verify that the radiometer is functioning nominally and that the backend processor is detecting and filtering RFI accurately. The final data product will include frequencies, unmitigated and mitigated brightness temperatures, latitude, longitude, pixel size, time, and RFI flags. These data products will be publicly available via web access to an OSU-hosted site (https://u.osu.edu/cuberrt/).

In summary, the CubeRRT system was designed to meet an ambitious set of mission objectives in order to demonstrate and mature spaceborne RFI mitigation technology. CubeRRT was launched in May 2018 on the OA-9 mission, docking successfully with ISS. Following deployment of CubeRRT into orbit, a commissioning phase will commence in which all systems are verified in advance of normal operations. Payload commanding will be developed by the OSU team, passed along to BCT for processing, and uplinked to CubeRRT via the WFF ground station. Downlinked telemetry and payload data will be received by WFF, processed initially by BCT, and handed off to OSU for ground-based verification of the RFI mitigation capability. A total of one year of mission operations is anticipated.

The information compiled and edited in this article was provided byHerbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (herb.kramer@gmx.net).